In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting material is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the light-emitting substance.
Such a light-emitting element is self-luminous elements and have advantages over liquid crystal displays, such as high visibility of the pixels and no need of backlight; thus, the light-emitting elements are thought to be suitable as flat panel display elements. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight, and has very fast response speed.
Furthermore, since such a light-emitting element can be formed in a film form, the light-emitting element makes it possible to provide planar light emission; thus, a large-area element can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to a lighting device and the like.
Such light-emitting elements utilizing electroluminescence can be broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, application of a voltage to the light-emitting element causes injection of electrons from a cathode and holes from an anode into the layer containing the organic compound having a light-emitting property and thus a current flows. The injected electrons and holes then lead the organic compound to its excited state, so that light emission is obtained from the excited organic compound.
The excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state (S*) is called fluorescence, and emission from the triplet excited state (T*) is called phosphorescence. Further, the statistical generation ratio of S* to T* in a light-emitting element is thought to be 1:3.
With a compound that can convert energy of a singlet excited state into light emission (hereinafter, called a fluorescent compound), only light emission from the singlet excited state (fluorescence) is observed and that from the triplet excited state (phosphorescence) is not observed, at room temperature. Accordingly, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
In contrast, an observation on a compound that can convert energy of a triplet excited state into light emission (hereinafter, called a phosphorescent compound) shows light emission from the triplet excited state (phosphorescence). Further, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs in a phosphorescent compound, the internal quantum efficiency can be theoretically increased to 100%. In other words, higher emission efficiency can be obtained than using a fluorescent compound. For this reason, light-emitting elements using a phosphorescent compound have been under active development recently in order that highly efficient light-emitting elements can be obtained.
When formed using the above-described phosphorescent compound, a light-emitting layer of a light-emitting element is often formed such that the phosphorescent compound is dispersed in a matrix containing another compound in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound. Here, the compound as the matrix is called a host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is called a guest material (dopant).
In the case where a phosphorescent compound is a guest material, a host material needs to have higher triplet excitation energy (energy difference between a ground state and a triplet excited state) than the phosphorescent compound.
Furthermore, since singlet excitation energy (energy difference between a ground state and a singlet excited state) is higher than triplet excitation energy, a substance that has high triplet excitation energy also has high singlet excitation energy. Therefore the above substance that has high triplet excitation energy is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance.
A compound including pyrimidine or the like as a partial structure has been studied as an electron-transport material or as a host material in the case where a phosphorescent compound is used as a guest material (e.g., Patent Document 1).
A compound in which a carbazole skeleton and a nitrogen-containing hetero aromatic ring are combined is disclosed as a host material in the case where a phosphorescent compound is used as a guest material (e.g., Patent Document 2).